This discussion is a continuation of one that began in a Nov. 27, 2021, article (link below) on the role that oil mist has played in protecting stored equipment for decades. Here, the focus is on a variety of advantages this technology offers in equipment preservation and elsewhere. Many times, oil-mist lubrication might not be considered because plant decision-makers don’t know about its simplicity, its relative low cost, and/or its technical merits. Why they don’t know is difficult to understand and certainly not worth speculating about. With the foregoing in mind, the next few paragraphs were written to inform teachable decision-makers.
50 YEARS OF OIL-MIST LUBRICATION
Since the late 1960s, oil mist has excelled as an unusually simple and highly dependable lube application method. Compared to traditional liquid oil sumps in pumps and the often-ill-defined methods of applying grease to electric motor bearings, plant-wide oil mist systems are much more reliable and cost-effective than dealing with individually liquid oil or grease lubricated machines. As of late 2021, an estimated 165,000 process pumps and 52,000 electric motors are being lubricated with oil mist. Well over 3,000 plant-wide oil mist systems are now in highly successful use. At their respective final points of application, usually at the bearing housings of a great variety of machines, the oil mist coalesces, i.e., it “plates out on” or coats all bearing surfaces.
The rate at which oil mist coalesces is greatly accelerated the very instant an oil and air mixture contacts bearing elements in motion. Small globules of atomized oil are then knocked together and form larger globs of oil; these are too heavy to remain suspended or carried along in air. As mentioned above, the globules of oil plate out on all bearing surfaces and provide an oil film that both lubricates and protects.
Especially notable is the protection of standstill equipment in facilities, where both an operating main pump and an installed standby spare pump sit side-by-side. The same, of course, is true for other machines equipped with rolling element bearings. Fans, small blowers, conveyors and other machines respond the same: Whenever bearings are not moving, fewer globules get knocked together and less oil will coalesce.
However, it only takes a small amount of oil to fully preserve the interior surfaces of a bearing’s housing. Even more important is the fact that the oil mist pressure inside a bearing’s housing is perhaps 0.1 psi greater than the pressure of the surrounding ambient air. Therefore, neither external water vapors nor particulates (e.g., dust, fine sand) can enter the bearing’s housing.
The capacity of an oil-mist system depends on bearing size and distance from the oil-mist generator (OMG). The OMG is a mixing valve or venturi (converging-diverging nozzle) where lube oil and clean instrument air are brought into contact with each other. There are no moving parts in the oil-mist generator. Level sensors associated with the oil reservoir are based on radar or infrared sensing technology; again, no moving parts.
From five to as many as 70 pumps and/or drivers can be connected to a single oil-mist system. The technology is widely used in advanced oil refineries and chemical plants, AKA “best in class” (BiC) facilities. When electric motor drivers are served in addition to pumps, and if the oil mist is captured after traveling through equipment bearings (capture makes this a closed system), this technology is cost justified, and most often yields payback periods of less than two years.
Here are some additional details on the technology:
- Due to its low maintenance requirements, pure-oil-mist lubrication, also called dry-sump oil mist, represents
both least risk and best available technology for lubricating
equipment incorporating rolling element bearings.
- Generally speaking, process pump sets with units up to 500 hp in size have been connected to oil-mist systems
on a plant-wide basis. However, many individual pumps and motors as large as 1,250 hp are known to operate
with this type of lubrication.
- Decades ago, a large industrial manufacturing company showed that its rolling-element-equipped electric
motors, some rated as high as 3,000 kW, could operate on pure oil mist.
ADVANTAGES OVER TRADITIONAL LUBE METHODS
Closed oil-mist systems allow little or no oil to escape into the surrounding atmosphere. Textile machine manufacturers were among the first to use closed-oil-mist systems in machines other than process pumps. As early as 1950, a Swiss textile machinery manufacturer relied on closed loop oil mist systems in hundreds of machines. Bearings in the high-speed draw rolls of its tire-cord production machines had to be kept cool and lubricated and only pure oil mist was up to the task.
That company, which has been in business since 1795, and a handful of others can attribute their longevity and success to intelligent decision-making. They have studied and embraced innovation long before the competition. No doubt, their early acceptance of oil-mist lubrication has contributed to their staying power. Likewise, recognizing and implementing superior lubrication methods plays an important part in pushing BiC oil refineries to the top. Understanding and optimizing lubrication play key roles in achieving years of high asset reliability and corporate profitability.
The following are the primary advantages of oil mist. It should be noted that none of these advantages have ever been disputed by knowledgeable professionals. Issues only arose when managers asked to see detailed cost justifications. However, obtaining payback within the first year of operation is not unusual for plant-wide systems of pumps and electric-motor drivers. (Editor’s Note: Those types of detailed cost justifications can be found in Heinz Bloch’s recent book, titled Optimized Equipment Lubrication, Oil Mist Technology and Full Standstill Protection, , DeGruyter, Berlin, Germany, ISBN 978-3-11-074934-2.)
- Pure-dry-sump-oil-mist systems contain no moving parts. Oil mist is applied without using oil rings.
(Oil rings are subject to abrasive wear and/or slowing down if the shaft system is not absolutely parallel.)
- With pure oil mist, service-intensive oil rings, constant-level lubricators, and traditional oil-refill-labor
requirements are eliminated.
- The labor requirement for pumps with oil mist has been estimated as 1/10th that of traditional lubrication.
- Since about 1998, the earlier practice of allowing excess oil mist to escape into the atmosphere has been
superseded by widespread use of closed systems.
- Closed systems avoid polluting the environment, with as much as 98% of the oil recovered for reuse. Some of
the earliest closed systems have been in highly successful service since the late 1950s and represent best-
available technology in all respect.
- Plant-wide systems are almost completely maintenance-free and fully self-checking. Users do not have to rely
on operators or maintenance workers to check and fill housings with oil or replace oil in bearings’ housings.
- Better lubrication conditions exist because the oil coating on the bearings is always new.
- Lower bearing-operating temperatures are routinely obtained. Reductions typically range from 10 to 20 degrees F
(or 6 to 13 degrees C).
- Power requirements are typically reduced by at least 1% (and sometimes even 3%) since bearings operate on a
thin oil film instead of plowing through a drag-inducing pool of oil or grease.
The extended mean time between failure (MTBF) benefits of oil mist over traditional liquid oil in sump lubrication have been well-documented. And oil mist was included in the venerable API 610 pump standard, 7th edition (1989), from the American Petroleum Institute (API). In describing the basic oil-mist-generation process, it should be noted that bearing housings contain no liquid oil. Instead, an oil-mist generator with no moving parts creates the oil mist in a central console.TRR
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX.
Tags: reliability, availability, maintenance, RAM, lubrication, lubricants, oil mist, pumps, electric motors, fans, blowers, conveyors, bearings, API 610